Optical communications systems typically include network nodes that can be connected by multiple concatenated optical components, including optical fibre spans (e.g., of 40-150 km in length) interconnected by optical amplifiers. Within each network node, optical signals are converted into electrical signals for signal regeneration and/or routing, and subsequently converted into optical signals for transmission to another network node.
The use of concatenated optical amplifiers within an optical path enables improved signal reach, i.e. the distance that an optical signal can be conveyed before having to be regenerated. However, signal degradation due to, e.g., noise, dispersion effects, non-linear effects and self-phase modulation increase as the optical signal propagates along the optical path and can become significant limiting factors of the signal reach.
One commonly used method of addressing the problem of dispersion in high-bandwidth communications systems is by inserting one or more optical dispersion compensators, within the optical path. Such dispersion compensators may, for example, take the form of length of fibre, a Mach Zehnder interferometer, an optical resonator, or a Bragg reflector. However the deployment of such optical dispersion compensators adds expense to the optical network.
Optical path chromatic dispersion compensation effected in the electrical domain of transmitters and receivers is described, e.g., in U.S. Patent Application Publication No. 2004/0067064 to McNicol et al., which is incorporated herein by reference. Such a technique reduces the cost of optical communication systems, as the costly optical dispersion compensators are not required.
Protection switching between two nodes for protecting against a signal failure in a given optical path is known in the art. One form of protection switching involves providing first and second distinct optical paths for respective first and second transmitter/receiver pairs for each of the two transmission directions. That is to say that four transmitters, four receivers and four optical paths are required for protection switching between two nodes. This is a costly proposition since transmitters and receivers are usually the most expensive parts of communications systems. Upon a signal failure being detected on a path, the client at the receiving end must enable the other of its two receivers and notify the transmitter end to enable its other transmitter.
Another form of prior art protection switching involves providing first and second distinct optical paths for a single transmitter/receiver pair for each of the two transmission directions, with the option of providing optical dispersion compensation in the paths. That is to say that two transmitters, two receivers and four optical paths are required for protection switching between two nodes. For each of the two transmission directions, a client is connected to a single transmitter and to a single receiver. The output of each transmitter of a given client is connected to a 1×2 splitter whose two outputs are connected via optical paths to a 2×1 optical switch, which is connected to the receiver of another client. Upon receiving a failed signal a first optical path, the client at the receiving end signals its switch to enable the other optical path. This is type of protection switching is termed unidirectional protection switching and requires a switching of paths at the receiving end only. This approach to protection switching is more affordable than the previously described approach.
However unidirectional protection switching is not possible in the case where dispersion compensation is effected at the transmitter end since the dispersion for which the transmitter compensates is typically different in the protection path compared to the working (i.e. main) path. Consequently, a single transmitter cannot transmit the same signal on both the working path and the protection path, while compensating for dispersion or other optical effects, as the dispersion is typically different in each path. Thus, such a unidirectional switching approach requires costly optical dispersion compensators deployed in each of the optical paths whenever dispersion compensation is required.
Therefore, it is desirable to provide affordable protection switching in communication systems having compensation enabled transmitters.